PPM Polarity

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Mike_K
Posts: 669
Joined: 16 Feb 2018, 06:35
Location: Hertfordshire

PPM Polarity

Post by Mike_K »

Hi All

I'm trying to finish off the documentation for a proportional encoder project that I've written and there is one thing that I'm not certain of, the correct description of the PPM polarity.

In the below screen-shot from my logic analyser of two ppm streams, the top one is typical of a Futaba and the bottom one typical of a JR transmitter. I've always called the top one negative polarity as it is mainly at ground and the bottom one positive polarity, but there are conflicting descriptions on the web.

To get a Spektrum DM9 module to work in a FrSky Tx you have to choose + polarity (and 400uS pulse), that would suggest the above description is correct. But if I put the waveforms into a Phil_G ppm monitor it says the opposite!! And Phil is not wrong very often.

I've got all the modules that I've tested to work OK (FrSky, OrangeRx, Spektrum, Corona, Multi-protocol, Futaba 35MHz and 2.4GHz and JR 35MHz), so I haven't a problem with the programming or the hardware, it's just I don't want someone saying I've got my descriptions wrong.

Thanks

Mike

ppm_polarity.jpg
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Flynn
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Joined: 17 Feb 2018, 14:48

Re: PPM Polarity

Post by Flynn »

My personal belief would side with Phil on this one.. the top trace shows positive going pulses so logically should be labeled as +PPM and the bottom trace shows negative going pulses so is -PPM....but I would be willing to accept there is a 50% chance I am wrong ;)
You only ever need two tools....WD40 and duct tape.
If it doesn't move when it should use the WD40 and if it moves and it shouldn't use the tape.
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Phil_G
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Re: PPM Polarity

Post by Phil_G »

The key word is 'going' Mike, your top trace shows 'positive going PPM' by long-established standards where 'polarity' is determined by the pulse itself, not the variable gap between.
Present day 'negative-going PPM' is as per your second pic but is in itself contradictory as it never goes negative like the old Kraft encoders used to. So the 'standard descriptions' are actually not very good.
Hence I personally describe your first pic as 'standing ground, positive going PPM' and your second as 'standing pos, ground-pulsed PPM' which is a bit of a mouthful. JR is generally 'standing battery pos, ground-pulsed PPM'. Then there's 'standing logic pos, ground-pulsed PPM' and vice versa, with 3v3 and 5v variations. All these descriptions are further confused by open-collector drive like Frsky, or almost-open-collector like futaba & hitec module ports, where the encoder doesnt provide any voltage at all, just a pulsed connection to ground. Futabas and Hitecs have opposing polarity on the trainer port and the module port, so you cant simply say Futaba PPM is pos or neg - you have to say where you're looking!

So its all a crock of poop really, but in a very general accepted sense, your pics show positive-going ppm and negative-going ppm respectively, like the tester says.
Standards are brilliant - so many to choose from :D


One of my many unpleasant attributes ;) is that I do enjoy the occasional bit of pedantry, and what we know and love as PPM in an RC transmitter sense (as implemented by Doug Spreng) has always been DPPM. No RC system takes its channel timing relative to the sync nor to a regenerated clock, its asyncronous and times relative to the previous pulse, hence its DPPM.
At the encoder end from the earliest multivibrator-followed by-halfshots encoder through NE5044 and to processor-based R/C ppm encoders, each pulse has always been timed relative to the previous one - often actually triggered by the previous one - so again, clearly DPPM 8-)

I think Futaba's biggest faux-pas is the reversed throttle channel, how thats not caused multiple accidents with electric powered models is pushing luck to the limit! Luckily all modern escs go into training mode, back in the day full throttle on power-up would give full throttle & ketchup everywhere :lol:

Scott - Mikes encoder is the absolute mutts nuts. It has everything any modern computer-radio should have and more. Its arduino-based with an OLED screen and pushbuttons to navigate the menus. Mike is very modest about it but it is simply superb!

Cheers
Phil
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Mike_K
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Re: PPM Polarity

Post by Mike_K »

Hi Phil and Flynn

Thanks both.

I think I'd be better put some pictures in the manual as well as words to explain the options. A picture they say is worth a thousand words and is "nearly" unambiguous. I say nearly because as Phil rightly points out it doesn't necessarily show if it is open collector or logic level and if so what the logic level is (battery, 5V, 3V3, CMOS or TTL etc). Aren't FrSky and OrangeRx modules wonderful, they will work with just about anything.

I also realise how the different manufacturers use different standards in so many other ways. My next section of the manual is the default output order/assignment or whatever the heck you want to call it. How did we finish up with TAER, AETR and AERT? And how did we finish up with so many different ways of describing end point adjust/travel volume etc. No wonder beginners to the hobby struggle with their first computer radios.

Thanks

Mike
Martin
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Re: PPM Polarity

Post by Martin »

Image

I like how Futaba have exponential the opposite way round to JR/Spektrum. I think in this case Futaba are most in the wrong as the expo you want to use in nearly every instance is the (Futaba) negative kind. Some JR and Spektrum sets don't even allow negative expo (which is Futaba positive expo).

Sanwa used to have their servo connector pins in a different order to everyone else. Eventually they gave up and swapped to 'standard' order, which meant that different generations of Sanwa receivers and servos were incompatible with each other!
Scott Todd
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Joined: 26 Mar 2018, 23:21

Re: PPM Polarity

Post by Scott Todd »

Mike,
Where do we find more about your Encoder?

Don't just tease us like that :)
Scott Todd
Posts: 66
Joined: 26 Mar 2018, 23:21

Re: PPM Polarity

Post by Scott Todd »

See...Now you are just teasing us MORE :)

I absolutely love your (Phil's) encoder. Its simplicity is perfect for older radios. I have already done several and have eyes on more.

The idea of integrating Arduino to a screen for a radio is super intriguing. I built Phil's analyzer and have been thru lots of tutorials on using them.

More info? inks?
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Mike_K
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Re: PPM Polarity

Post by Mike_K »

Hi Scott

I didn't intend to tease you, I just assumed most people on the forum have seen one of my transmitter conversions and I didn't want to bore everybody by repeatedly posting every little modification. If you were at last years Ponty meeting or the impromptu Buckminster meet later in the year you will have seen me flying my converted Skyleader and I have posted pictures of it somewhere on this forum. Or was it the previous forum?

As Phil has kindly said, I have been developing the encoder over the last two years to the point that it can do pretty much the same as many mid range transmitters from a few years ago, something similar to a first generation Spektrum DX7 or DX8, but it has single channel compound and sequential emulation in addition. As there has been some interest, I hope to sell ready made and tested boards to make it feasible for non electronic enthusiasts to convert a set, but when I'm finished I will also release the circuits and code for those who like to do a bit of DIY electronics.

The encoder firmware is complete (except for a few minor mods so it can display up to 13.5V so it will work with 35MHz modules) and I've been busy for the last six months writing the user manual, a quick start manual and a construction manual. Shaun Garrity has kindly offered to proof read the manuals, but as he's a busy man I don't expect them to be read for a month or two, which will give me a chance to finalise the "production" circuit boards.

If you are interested in what it can do (and what it can't do), I've uploaded the draft user manual to my website so you can have a look. But bear in mind that it is a draft document and that my grammar is not the best - that's why Shaun is going to help out!

http://www.cavaya.co.uk/download/Encode ... .3.21a.pdf

The hardware is based on an Arduino Nano. In the end, the meagre resources of the Nano has been the limiting factor, I have so many ideas of other things I'd like to include but can't because I've run out of flash memory. When you start, 32K of flash seems enormous, but it isn't long before it's all used up. And every single byte of EEPROM has been used. I've already started on a MK2 version based on a STM32, but I'm back on a steep learning curve with that processor, so don't hold your breath :)

I have converted a Skyleader SLX and Futaba M6 for myself, I gave Phil and Shaun a Futaba M6 each last year at Ponty and have done other Futaba, Skyleader and an OS Cougar conversion for friends. I started a Micron PL-7D conversion before Christmas, but other projects such as 3D printing OS Pixie cases, my own single channel encoder, converting a Graupner Grundig Variophon reed/tone set and building a few planes have got in the way.

Cheers

Mike

PS What a brilliant flying day it's been, the best of the year so far. I've had to come in for a rest from the sun

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